Pathogens resistant to multiple antibiotics present a continually increasing health risk, particularly in clinical settings. Patients may acquire infections through intrusive, but necessary, medical means, such as infections in the respiratory pathway during mechanical ventilation, in the urinary tract or blood vessels via catheters or even through skin wounds, such as incisions required for any number of medical procedures. Immunocompromised patients and patients located in Intensive Care Units (ICUs) are at increased risk of acquiring nosocomial infectious diseases which may be resistant to one or more antibiotics. For a variety of reasons, such infections may be associated with a high mortality rate. Previously, the European Center for Disease Control (ECDC) reported 25,000 annual deaths due to multi-resistant pathogens.
Well-selected, early antibiotic treatments provide the best defense against such multi-resistant pathogens. Given the high prevalence of resistances, current procedures require a bacterial culture for identification of the microorganism followed by an antibiogram, which routinely requires 2-3 days of bacterial growth. The step of culturing bacteria to construct an antibiogram alone generally requires about one day of incubation, or about a minimum of 18 hours.
Given the relative long time necessary to perform standard antibiogram, antibiotics are usually empirically provided at the onset. This first line of defense often relies on antibiotics generally known to be effective based on the likely pathogen involved. However, such treatments may be ineffective in 20-40% of cases, and a change of antibiotics later may have a reduced probability of success. Even educated assumptions may contribute to antibiotic misuse or overuse resulting in increasingly resistant strains of bacteria while the results of an antibiogram are pending.
The antibiogram results from clinical testing of isolated bacteria strains in vitro for bacterial susceptibility to antibiotics. A common methodology for constructing an antibiogram based on diffusion is the Kirby-Bauer method (Bauer A W, Kirby W M M, Sherris J C, Turck M Antibiotic susceptibility testing by a standardized single disc method. Am J Clin Pathol 1966; 45:493-496). In the semi-quantitative Kirby-Bauer method, several discs containing different antibiotics are placed in different zones of nutrient rich bacteria culture. Because the antibiotic diffuses into the agar away from the disc, the diameter around the disc in which bacteria does not grow is suggestive of the minimum inhibitory concentration (MIC) of that antibiotic to the cultured strain of bacteria.
A quantitative method may rely on dilution in a series of broths or agar solutions having progressively lower concentrations of the antibiotic in question. The lowest concentration of antibiotic in which the bacteria cannot grow provides the minimum inhibitory concentration of that antibiotic to the tested strain of bacteria. This quantitative method may be routinely employed in the hospitals, usually using commercial panels of antibiotics and semi-automated systems of incubation and software for data interpretation like the MicroScan WalkAway™ (Siemens), Phoenix (Becton Dickinson), or Vitek™ 2 (bioMérieux). With such growth-dependent automated systems, results of susceptibility or resistance to antimicrobians from a specific microorganism may be obtained in around 6-9 hours.
Each of the diffusion and the dilution methods rely on the principal of inhibiting bacterial proliferation in a nutrient rich medium and this requires sufficient time for many reproductive cycles of bacteria. As such, both methodologies may require a minimum of between 18 hours and 24 hours. It can be understood, conventional testing such as antibiograms fails to address the problems described above.
Additionally, a number of experimental approaches have been attempted with the goal of achieving faster susceptibility-resistance determinations. However, those experimental approaches failed to supplant the conventional, time consuming antibiogram. Accordingly, a need still exists for susceptibility testing capable of rapidly determining an antibiotic treatment enabling the rapid, effective administration of effective antibiotic treatments and reducing the misuse or overuse of antibiotics.